Purple-leaf plum is the small dark-leaved ornamental tree sold under several names, and the single decision that determines whether it stays that colour is where it is planted. The anthocyanin pigment that masks the green is produced in strong light, so a tree in shade is a green tree with a purple cast, and no amount of feeding changes it.
Position and soil
Plant it in full sun. Not part-day, not the bright side of a wall shadowed by a mature tree, since the shade of a big neighbour is the most common reason these trees fade. It is a warm-climate plant that is not particularly hardy when young, so a sheltered site with sun is the honest requirement.
Soil is the easy part. Deep, fertile, well-drained sandy loam is best; a neutral to slightly acid profile suits it, and heavy clay will carry it provided drainage is reasonable. The root system will not tolerate a waterlogged profile at any point.
Water, and the autumn control that protects the tree
Through the growing season this tree likes moisture and should be watered properly rather than sprinkled. Once rain becomes frequent, stop supplementing it and see that the ground drains: water held around the roots in the wet season is a genuine risk to the tree, not a precaution.
From autumn, watering must be cut back. The reason is not drought tolerance. Wet autumn soil pushes a flush of late soft growth that the tree then cannot harden off before cold weather, and that growth is what dies back in winter and pulls the shape apart. A dry autumn is a hardy winter, and it is one of the very few genuinely counterintuitive watering rules on a garden tree.
Feeding: once a year, and that is the limit
One organic application in late autumn, before the ground closes for winter, is enough for a tree in reasonable soil. This is the point at which most growers overdo it. Rich nitrogen feeding makes the leaves grow fast, larger and darker green, and it destroys the thing you planted the tree for: the colour goes muddy, loses its brightness, and the ornamental value drops. A purple-leaf plum is a plant you feed to restrain rather than to encourage.
Shape, propagation and what eats it
Two shapes serve the tree. Open-centred, with a short trunk and three to five main limbs spreading to a vase, is the one that keeps light into the interior; modified leader is the alternative for a tree that wants a central stem. When forming it, keep some side branches at each tier so the canopy fills rather than showing bare framework, and once the shape exists, the annual job is small: remove crossing, downward, dead and congested wood. It flowers on the previous season's growth, so heavy winter cutting costs you the spring blossom.
Plants are raised from cuttings, chip or T-bud grafting onto a plum or myrobalan rootstock, and air layering, with grafting the usual commercial route. Layering keeps a named variety on its own roots, which is useful where a grafted tree above a sucker-producing rootstock becomes a nuisance. The underlying rules of propagation from a stem are covered in what a cutting is. A full hardwood protocol, with the bud counts, the soaking and the heeling-in stage spelled out, is in the white-twig dogwood method.
Watch for three things. Spider mites run on the underside of the leaves in hot dry weather and bleach the colour out. Caterpillars, including bagworms and slug caterpillars, strip foliage fast enough to matter. The disease worth naming is bacterial shot hole, which produces small dark spots that drop out and leave perforated leaves; it is worst in wet springs, it weakens rather than kills, and the control is copper sprays at bud break plus sanitation, keeping the leaf litter cleared, because the bacterium carries over on fallen leaves. Sap-suckers across those two groups are dealt with in what to do about aphids in pots.
